1. Technical Field
The present invention relates to a quantum interference device, an atomic oscillator and a magnetic sensor.
2. Related Art
An atomic oscillator based on an electromagnetically induced transparency (EIT) system (also called a coherent population trapping (CPT) system) is an oscillator using a phenomenon (EIT phenomenon) in which when two resonant lights different from each other in wavelength (frequency) are simultaneously irradiated to an alkali metal atom, the absorption of the two resonant lights is stopped. It is known that the interaction mechanism between the alkali metal atom and the two resonant lights can be explained in a Λ-type 3-level system model. The alkali metal atom has two ground levels, and when resonant light 1 having a wavelength (frequency) corresponding to an energy difference between the ground level 1 and the excited level or resonant light 2 having a wavelength (frequency) corresponding to an energy difference between the ground level 2 and the excited level are individually irradiated to the alkali metal atom, light absorption occurs as is well known. However, when the resonant light 1 and the resonant light 2 in which the frequency difference accurately coincides with the frequency corresponding to the energy difference between the ground level 1 and the ground level 2 are simultaneously irradiated to the alkali metal atom, a superposition state of the two ground levels, that is, a quantum interference state occurs, the excitation to the excited level is stopped, and the transparency (EIT) phenomenon occurs in which the resonant light 1 and the resonant light 1 pass through the alkali metal atom. This phenomenon is used and an oscillator with high accuracy can be formed by detecting and controlling the abrupt change of light absorption behavior when the frequency difference between the resonant light 1 and the resonant light 2 shifts from the frequency corresponding to the energy difference between the ground level 1 and the ground level 2. Besides, since the energy difference between the ground level 1 and the ground level 2 is sensitively changed by the intensity or fluctuation of external magnetic field, a highly sensitive magnetic sensor can also be formed by using the EIT phenomenon.
In the atomic oscillator or the magnetic sensor, in order to improve the signal to noise ratio (S/N ratio) of the output signal, the number of alkali metal atoms that cause the EIT phenomenon has only to be increased. For example, JP-A-2004-96410 (patent document 1) discloses a method in which in order to improve the S/N ratio of an output signal of an atomic oscillator, the thickness of a cell in which gaseous alkali metal atoms are confined is increased, or the beam diameter of a laser beam incident on the cell is increased. In either method, in order to widen an area where the alkali metal atoms are irradiated with the resonant light, the thickness or the height of the cell is increased. Besides, U.S. Pat. No. 6,359,916 (patent document 2) proposes an atomic oscillator in which D1 line is used as a light source, so that the intensity of an EIT signal (signal of light passing through the alkali metal atom by the EIT phenomenon) is improved theoretically as compared with the related art case of D2 line, and the sensitivity and frequency stability accuracy is improved by this. In the atomic oscillator disclosed in the patent document 1 or the patent document 2, only one pair of two kinds of laser lights satisfying the occurring condition of the EIT phenomenon is used.
When attention is paid to individual atoms constituting a group of gaseous alkali metal atoms in the cell, they have a certain velocity distribution corresponding to the respective motion states. FIG. 15 is a schematic view of the velocity distribution of a gaseous alkali metal atom group confined in a container. The horizontal axis of FIG. 15 indicates the velocity of a gaseous alkali metal atom, and the vertical axis indicates the ratio of the number of gaseous alkali metal atoms having the velocity. As shown in FIG. 15, the gaseous alkali metal atoms have the certain velocity distribution corresponding to temperature, the center of which is the velocity of 0. Here, the velocity means an atom velocity component parallel to the irradiation direction when a laser beam is irradiated to the gaseous alkali metal atom group, and the value of the velocity of the atom at rest relative to a light source is 0. As stated above, when the velocity of the gaseous alkali metal atoms has the distribution, by the light Doppler effect (Doppler shift), the apparent wavelength (frequency) of the resonant light, that is, the wavelength (frequency) of the resonant light when viewed from the gaseous alkali metal atom has a distribution. This means that in atoms different in velocity, the excited levels are seemingly different. As shown in FIG. 16, the excited level has the broadening of certain width (Doppler broadening). Accordingly, even if a pair of the resonant light 1 and the resonant light 2 are simultaneously irradiated, only a very small part of the atoms having a specific velocity component value (for example, 0) with respect to the incident direction of the laser can actually cause the EIT phenomenon. A considerable number of gaseous alkali metal atoms which do not cause the EIT phenomenon and remain exist in the group, and the ratio of atoms contributing to the EIT phenomenon is very low. In order to increase the intensity of the EIT signal in the state where the EIT occurrence efficiency is low as described above, the thickness or the height of the cell must be increased as disclosed in the patent document 1, and there is a problem that miniaturization is difficult. Besides, in the state where the EIT occurrence efficiency is low, since the use efficiency of laser power is low, when the intensity of the EIT signal is kept at a certain level or higher, it is difficult to reduce the laser power, and there is a disadvantage also in power saving.